1. Field of the Invention
The present invention relates to a control circuit and a control method for a matrix type display device capable of providing gray scale display.
2. Description of the Related Art
A matrix type display device is used in various office automation equipment such as personal computers and word processors, multimedia information terminals, audio-visual equipment, game machines, and the like. Recently, a matrix type display device which can provide gray scale display is often used.
To provide gray scale, a frame modulation system or a pulse width modulation system is widely used in a control circuit of a conventional display device.
In the frame modulation system, a constant ON or OFF display voltage which is to be applied to each pixel is selected in a frame-by-frame basis, depending on a gray level which the pixel is to display. The gray level of a pixel is determined by the temporal average of the number of frames at which the ON display voltage is applied to the pixel. In this manner, gray scale display having two or more levels can be performed.
In the pulse width modulation system, the width of a pulse applied to each pixel is modulated depending on a gray level which the pixel is to display. In this manner, gray scale display having two or more levels can be performed.
Japanese Laid-Open Publication No. 2-1812 discloses a method in which gray scale obtained by the pulse width modulation is further subjected to the frame modulation.
The frame modulation system, however, poses the following problem. To provide a given number of levels of gray scale, the necessary number of frames is at least (the number of levels-1). Therefore, the number of frames increases in proportion to the number of levels of gray scale. The increased number of frames leads to a significant flicker or waving in a display. For this reason, when the frame modulation system is used for a liquid crystal panel having high speed response, for example, the problem becomes more significant. To avoid the problem, the maximum number of frames is around four in practical use.
The pulse width modulation system needs to create a pulse corresponding to a given gray level within a period of one horizontal scanning time. Accordingly, the number of times that a data signal changes is more than when the gray scale display is not required. For this reason, the frequency of the data voltage signal becomes higher, resulting in the significant rounding of the data voltage signal caused by electrode resistance and liquid crystal capacity and the wave-form distortion of a scanning voltage induced by a data voltage. In this case, a root-mean-square (RMS) value of voltage whose value is different from the RMS value of the original voltage is applied to liquid crystal, which leads to a reduction in display quality, such as crosstalk.
The above-described problem on the pulse width modulation system still remains in the method disclosed in the above-described Japanese Laid-Open Publication No. 2-1812 where gray scale obtained by the pulse width modulation is further subjected to the frame modulation.
The above-described problems will be described in greater detail with reference to FIGS. 6 to 8 below.
For example, a display device includes a liquid crystal panel 600 with a 4 by 4 matrix of pixels as shown in FIG. 6. The liquid crystal panel 600 includes column electrodes X1 to X4 and row electrodes Y1 to Y4. Pixels P11 to P44 are defined by points of intersection of the column electrodes X1 to X4 and the row electrodes Y1 to Y4.
FIG. 7 shows, for example, patterns of gray levels of pixels in frames of the display device when all the pixels display a gray level of {fraction (5/60)} using a conventional driving system. FIG. 8 shows driving waveforms XW1c to XW4c for the column electrodes X1 to X4, and driving waveforms YW1c to YW4c for the row electrodes Y1 to Y4.
Each frame displays 16-level gray scale ranging from a gray level of {fraction (0/15)} to a gray level of {fraction (15/15)} using the pulse width modulation system. The pattern of gray levels is rearranged for each frame in a period of 4 frames using the frame modulation system. As a result, the display device can display 61-level gray scale ranging from a gray level of {fraction (0/60)} to a gray level of {fraction (60/60)}.
As can be seen from FIG. 7, the pixel P11 at the point of intersection of the column electrode X1 and the row electrode Y1 displays a gray level of {fraction (2/15)} at the first frame and a gray level of {fraction (1/15)} at the second to fourth frames, resulting in a gray level of {fraction (5/60)}. Each frame displays a different pattern of gray scale of pixels, but every pixel can display a gray level of {fraction (5/60)} using 4 frames.
As can be seen from FIG. 8, all the driving waveforms XW1c to XW4c applied to the column electrodes X1 to X4 have higher frequency than when gray scale display is not required. This leads to an increase in the rounding of a waveform which occurs due to electrode resistance and liquid crystal capacity every time when the waveform changes. As a result, the actual waveform significantly differs from the ideal waveform that has no rounding. The driving waveforms YW1c to YW4c applied to the row electrodes Y1 to Y4 are distorted when the driving waveform applied to the column electrode changes. This is because the change induces the waveform distortion.
The higher frequency the driving waveform applied to the column electrode has, the more number of times the waveform changes, resulting in an increased rate of waveform distortion. The amplitude of the waveform distortion becomes larger as the number of column electrodes which change the waveforms thereof at the same time increases. As shown in FIG. 8, since the number of column electrodes which change the waveforms thereof at the same time is great, the amplitude of the waveform distortion is large.
Each pixel receives the addition of the driving waveform applied to the column electrode and the driving waveform applied to the row electrode. Therefore, a voltage waveform which is actually applied to each pixel includes both waveform rounding and waveform distortion. As a result, the actual waveform significantly differs from the ideal voltage waveform. Accordingly, the RMS value of a voltage becomes much different from the ideal value.
In a display device using the conventional driving system, when the number of column electrodes is, for example, several hundred, the difference between the RMS value of a voltage and the ideal value varies greatly between each column electrode. This leads to a reduction in display quality, such as crosstalk.